1. Field of the Invention
The present invention is concerned with the field of power electronics. It relates to an invertor comprising a plurality of invertor bridges, which are operated in parallel on the same DC voltage intermediate circuit and whose output voltages are summed via a transformer, which transformer has a number of primary windings and associated secondary windings which corresponds to the number of invertor bridges, each invertor bridge being respectively connected on the output side to a primary winding, and the secondary windings being connected in series for the purpose of summing the output voltages, and the invertor bridges each being driven with pulse duration modulation according to an auxiliary control voltage, and the auxiliary control voltages of the individual invertor bridges having a constant phase difference between one another, and the transformer having a center tap, which is grounded via a ground connection.
2. Discussion of Background
In order to connect electricity supply systems having a different number of phases and/or AC voltage frequency, such as e.g. between a 50 Hz three-phase power supply system and a single-phase 16 2/3 Hz railway grid, use is increasingly being made of solid-state couplings and railway power converters which are equipped with power semiconductors and are often designed as converters having a DC voltage intermediate circuit. In accordance with FIG. 1, such a railway power converter 10 comprises, for example, a (thyristor-equipped) converter 13 which draws the three-phase current from the three-phase power supply system 11 via a transformer 12 and converts it into a direct current, a DC voltage intermediate circuit 14 for smoothing and/or buffer-storage, and an invertor 15 which converts the direct current back into an alternating current at the desired frequency and feeds it into the railway grid 16.
In the invertor 15, use is usually made of one or more invertor bridges, operating in parallel, with switchable valves (e.g. GTOs), which are driven with pulse duration modulation and approximate the desired sinusoidal output voltage by a sequence of duration-modulated square-wave pulses of alternating polarity. A triangular-waveform auxiliary control voltage is usually used in this case for the pulse duration modulation. Details about the driving can be found for example in an offprint (No. 9608-1000-0) from the applicant "Vollstatische 100-MW-Frequenzkupplung Bremen" [Solid-state 100 MW frequency coupling Bremen]. If a plurality of invertor bridges are operated in parallel, the output voltages are summed. A reduction in the harmonic content is achieved by driving the individual invertor bridges via the auxiliary control voltages in a phase-shifted manner.
An example of the structure of an invertor 15 is represented in FIG. 2. The invertor 15 of this example comprises eight invertor bridges B1, . . . ,B8 which operate in parallel and, with a respective capacitor C1, . . . ,C8 in parallel at the inputs, are connected to the input lines 17, 18 coming from the DC voltage intermediate circuit 14. A transformer 19 is provided for the purpose of summing the output voltages of the invertor bridges B1, . . . ,B8, which transformer contains a winding pair comprising a primary winding P1, . . . ,P8 and a secondary winding S1, . . . ,S8 for each of the invertor bridges B1, . . . ,B8. The outputs of the invertor bridges B1, . . . ,B8 are respectively connected to the corresponding primary windings P1, . . . ,P8; the secondary windings S1, . . . ,S8 are connected in series. The summed output signal is available on the output lines 20, 21. In order to suppress harmonics, the transformer 19 may additionally be equipped with tertiary windings T1, . . . T8, which are connected in series and are damped by a corresponding filter circuit 25 (in this respect see, for example, EP-B1-0 149 169). Examples of duration-modulated and phase-shifted pulse trains for the invertor bridges B1, . . . ,B8 are represented in FIG. 3. Summation of the individual pulse trains in the transformer 19 produces therefrom the resultant summation voltage u.sub.Bi in FIG. 4.
Problems with the type of invertor illustrated in FIG. 2 arise if--as is necessary in the case of some railway grids--the transformer 19 of the invertor 15 is grounded at a center tap 23 by a ground connection 24 via a resistor 22 (or else without a resistor, that is to say in "hard" fashion) (see FIG. 2). These problems may be illustrated with reference to the equivalent circuit diagrams represented in FIGS. 5 to 8: The invertor, which operates as a voltage source converter (Voltage Source Converter, VSC), can be described in principle (FIG. 5) by a voltage source 26 having the voltage u.sub.Bi which drives a corresponding current i.sub.Bi through a circuit formed by the impedances 27,28 and 29. The impedances 27 and 28 with the values z.sub.1 and z.sub.2, respectively, represent the transformer 19 and the impedance 29 with the value z.sub.3 represents the filter circuit 25. The railway grid 16 can be described in the equivalent circuit diagram by the impedance 30 (z.sub.4) and the voltage source 31.
As a result of the grounding (via the resistor 22) at the center tap 23 of the transformer 19, the equivalent circuit diagram of the VSC from FIG. 5 can be converted into an equivalent circuit diagram in accordance with FIG. 6. The voltage source 26 is in this case divided into two voltage sources 32 and 33 having the partial voltages u.sub.Bi,a and u.sub.Bi,b, where: EQU u.sub.Bi =u.sub.Bi,a -u.sub.Bi,b (1)
The impedances 27 and 28 of the transformer 19 are now divided in FIG. 6 into impedances 34 and 39 and, respectively, 35 and 40, in each case having half the original impedance value, namely z.sub.1 /2 and z.sub.2 /2. The impedance 29 with the value z.sub.3 is preserved while the impedance 30 and the voltage source 31 of the railway grid 16 are likewise divided into the impedances 36 and 41 (in each case having the value z.sub.4 /2) and, respectively, voltages sources 42 and 43. The grounding via the center tap 23 of the transformer 19 is represented by the resistor 37 having the value R.sub.E in FIG. 6. A corresponding resistor 38 having the value R.sub.E,r describes the total remote grounding resistance of the railway grid 16.
According to the concept of modal decomposition, the equivalent circuit diagram of FIG. 6 can be decomposed into two superposed subsystems, namely into the common mode system and the differential mode system. The two superposed systems can then be treated separately from one another and the resultant currents and voltages simply added at the end of the analysis in order to obtain the real physical quantities. The equivalent circuit diagram in the common mode system for the upper half of the VSC is represented in FIG. 7. In addition to the already known impedances 34, 35 and 36, the circuit contains the resistors 45 and 46, which each amount to twice the grounding resistors 37 and 38, respectively. The voltage source 44 outputs a voltage u.sub.Bi,CM which drives a current i.sub.Bi,CM through the circuit. The equivalent circuit diagram in the differential mode system for the upper half of the VSC is illustrated in FIG. 8. In addition to the already known impedances 34, 35 and 36, the impedance 48 is present here as well, which impedance corresponds to half the impedance 29 and is characteristic of the filter circuit 25. The voltage source 47 outputs a voltage u.sub.Bi,D which drives a current i.sub.Bi,D through the circuit.
The following relationship emerges for the voltages and currents: EQU u.sub.Bi,a =u.sub.Bi,CM +u.sub.Bi,D (2) EQU u.sub.Bi,b =u.sub.Bi,CM -u.sub.Bi,D (3)
and also EQU i.sub.Bi =i.sub.Bi,CM +i.sub.Bi,D (4)
and EQU i.sub.E =2*I.sub.Bi,CM. (5)
It is immediately evident from FIGS. 5 to 8 and equations (1) to (5) that the common mode voltage u.sub.Bi,CM is undesirable because it drives a common mode current i.sub.Bi,CM which can flow back only through the grounding resistors 37 and 38. The level of the common mode current i.sub.Bi,CM is primarily limited by the impedances z.sub.1 and z.sub.2 of the transformer 19. The common mode current i.sub.Bi,CM has two disadvantageous effects:
It causes considerable losses both in the local grounding resistor 22 or 37 and in the remote grounding resistor 38. As shown by simulations of a real plant (approximately 50 MW), the losses in the grounding resistor 22 (at a nominal resistance of R.sub.E =334 .OMEGA.) can be approximately 50 kW and are therefore of an unacceptable order of magnitude. PA0 In the railway grid (for example a 138 kV grid), it can cause interference in adjacent communications equipment.